Copy the files as user sage

Author: nthiery

Dockerfile

@@ -1,4 +1,4 @@
 FROM sagemath/sagemath:8.0-2
 # Inspired from https://mybinder.readthedocs.io/en/latest/dockerfile.html#preparing-your-dockerfile
 # Make sure the contents of our repo are in ${HOME}
-COPY . ${HOME}
+COPY --chown=sage:sage . ${HOME}